The present invention relates to a resin composition heat curable into products excellent in heat resistance, which is useful for film-shaped adhesive, lamination materials, molding compounds, and so forth.
Most of the polyimides used hitherto for heat resistant adhesives, lamination materials, etc. have been made from bismaleimide and 4,4'-diaminodiphenylmethane. Compositions based on this type of polyimide are handled in powder form because of the low storage stability of their solutions. Hence, when used for such purposes as mentioned above, these compositions are dissolved in solvents before processing.
However, such a composition itself cannot be cast into films because of its hard and brittle nature. When it is used for lamination materials and the like, its solution is prepared, and supports or reinforcements such as glass cloths are impregnated with the solution and then dried by heating to form prepregs. Accordingly, it is impossible to produce thin lamination materials or the like, since the thinness of such prepregs is limited by the thickness of the supports
Japanese Patent Publication No. 48-18960 discloses polyethersulfoneimide. This imide resin has a high softening point and hence involves a problem of its low bonding strength and high degree of thermal shrinkage.
On the other hand, Japanese patent application Laid-Open Ser. No. 56-161457 discloses a polyamide-acid resin composition containing a maleimide compound. However, compositions based on polyamide-acid resins have drawbacks in that high temperatures are required for the ring closure and the condensation by-product water tends to form voids in the molded articles.
Further, Japanese patent application Laid-Open Ser. Nos. 55-123622 and 59-179558 disclose polyimide resin compositions containing maleimide compounds. When polyimide resins are employed, the problem of the water formation by condensation does not arise but the polyimides exemplified in the patent applications have high softening points so that large amounts of maleimide compounds need to be added in order to lower molding temperatures of the compositions. As a result, the resin compositions become brittle and hence cannot be used in any form but in the form of materials reinforced with glass cloths or the like and in the form of powders for molding purposes When the contents of maleimide compounds are low, it is necessary for the production of prepregs to leave large amounts of solvents in the prepregs in order to lower the molding temperatures thereof, since the softening points are high. This raises the problem of the formation of voids in the molded products.